If you aren't interested in why this works just the Introduction to the RFC explains the intent, specifically what we care about here is that TLS delivers:
"Integrity: Data sent over the channel after establishment cannot be modified by attackers without detection."
And further notes "These properties should be true even in the face of an attacker who has complete control of the network".
https://tools.ietf.org/html/rfc8446
If you aren't interested in why this works just the Introduction to the RFC explains the intent, specifically what we care about here is that TLS delivers:
"Integrity: Data sent over the channel after establishment cannot be modified by attackers without detection."
And further notes "These properties should be true even in the face of an attacker who has complete control of the network".